Humans in the clear over mammoth extinction?

You might say it was a case of kicking the woolly mammoth while it was down.

Arguments have raged over whether climate change or hunting by humans is what drove the beasts to extinction and the answer, it seems, is a bit of both. Climate change decimated the last remaining populations and hunters delivered the final coup de grâce.

The woolly mammoth once roamed freely in the cold, dry steppes of Siberia and North America, but disappeared some 3000 years ago.

Some palaeontologists say the warmer temperatures brought on by natural climate change gradually pushed the species off the map. Others contend that humans broke up the populations and hunted them to extinction.

Now a team led by David Nogués-Bravo of the National Museum of Natural Sciences in Madrid, Spain, say they have evidence that humans do not have much blood on their hands after all.

Steppe specialist

The researchers used computer models to determine what parts of the globe had "mammoth-friendly" climes between 126,000 and 6,000 years ago. They also generated computer models of the extent of human populations at the time.

The study suggests that mammoths were already in dire straits 126,000 years ago, when warm temperatures had forced them all the way to the Arctic Circle. But they survived the crunch and as temperatures dropped again, their range once more expanded southwards.

By 6000 years ago, temperatures had once more risen dramatically and the forests that ringed the Mediterranean had moved northwards.

"The woolly mammoth was a specialist in the steppes and would not have been able to survive in the warmer, more humid forest," says Nogués-Bravo.

As a result, the mammoths' habitat was reduced to an Arctic territory barely bigger than it had been some 120,000 years earlier. But this time they had to contend with a new threat - humans.

Hunt evidence

Given estimates of human and mammoth populations at the time, Nogués-Bravo and his colleagues estimate that humans would only have had to kill one mammoth each every three years at the most to push the species to extinction.

"There is little direct evidence of humans hunting woolly mammoths," says Nogués-Bravo. For instance, what looks like traces of arrowhead impacts have been observed on mammoth bones, but the evidence is contentious.

"This study is a real contribution to the debate," says Ross MacPhee of the Natural History Museum in New York. "It can't be correct that humans arrive on the scene and suddenly all the megafauna put their toes up."

'Nail in the coffin'

He cites evidence of humans in the Arctic Circle about 20,000 years ago, some 10,000 years before the mammoth population appears to have suffered a massive die-off.

MacPhee welcomes Nogués-Bravo's conclusion that climate change must have played a greater part in the disappearance of mammoth, with humans hammering the final nail in the coffin.

"What we are saying is that because of the climate-driven collapse of the population, it would have taken very little for humans to finally kill them off," says Nogués-Bravo.

He says new ways of more accurately studying by-gone mammoth populations using DNA may soon lend more support to his findings.

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